"We designed nanoscale triplex layers - less than 20 nm thick - which possess unusual, nonlinear nanomechanical responses," Vladimir Tsukruk, a professor at Iowa State University, told nanotechweb.org. "[The layers] feel hard for very small deformations, rubbery for medium deformations, and hard again for large deformations."
The coatings were deposited onto bare silicon surfaces coated with an epoxy-terminated self-assembled monolayer (SAM). At the core of the coatings was an 8 nm-thick compliant rubber layer sandwiched between two hard layers. The rubber layer provided energy dissipation under load while the top hard layer prevented the penetration of asperities through the rubber layer and also lowered friction. The composite modulus of the coating was 1 GPa.
Currently, designers protect materials such as silicon against surface damage with thin organic/polymeric coatings. However, Tsukruk says that these coatings cannot handle deformations of several nanometres or heal local damage, and they exhibit a simple linear response to external load.
The team's trilayer coatings were able to reversibly sustain deformations of as high as 60% compression. The scientists claim that the wear stability of their coatings are better than that of conventional SAM-modified surfaces by several orders of magnitude. They tested the behaviour of the coating by nanomechanical probing with a sharp scanning probe microscope (SPM) tip. In one test, the trilayer surface was worn down after 3000-3500 cycles whereas an alkyl-silane SAM failed after 900 cycles.
Now the researchers plan to optimize the design to increase its tribochemical resistance, and modify the fabrication technology so that it is compatible with MEMS surfaces such as polysilicon.
The work is published in Applied Physics Letters.